Calibration standards, mehtods, and kits for water determination

ABSTRACT

The invention relates to a formed tablet calibration standard-reagent for calibrating Karl Fischer reactions for determining water content in a substance, said reagent containing a first component, namely sodium tartrate dihydrate, and a second component, namely magnesium stearate. The invention further relates to a method for determining the water content of a substance using a Karl Fischer analysis. The invention further relates to a formed tablet calibration standard-reagent kit, comprising a sealed package containing said formed tablet calibration standard-reagent.

BACKGROUND OF THE INVENTION

[0001] This invention relates to improved calibration reagents for waterdetermination using the Karl Fischer reaction. More particularly, theinvention relates to a formed tablet calibration standard-reagent forcalibrating Karl Fischer reactions for determining water content in asubstance, said reagent containing a first component, namely sodiumtartrate dihydrate, and a second component, namely magnesium stearate.

[0002] Moisture measurement is valuable because the presence of watercan adversely affect a variety of applications across multipleindustries. Some examples include pharmaceutical drug stability;foodstuff storage quality; properties of oils (e.g. viscosity); andreduced chemical reaction yield (e.g. production of plastics). Moisturecontent determination is an evaluation criterion necessary for stabilityconsiderations of New Drug Applications. Accurate control and monitoringof moisture in these fields is often required by regulatory agencies andnecessary to improve product quality.

[0003] A number of chromatographic, spectroscopic, electronic, thermal,and wet chemical methods have been used in the past to determinemoisture levels (S. K. MacLeod, Anal. Chem., 1991, 63, 557A-565A). Themost common of these are loss on drying (LOD), thermogravimetricanalysis (TGA), gas chromatography using a thermal conductivitydetector, and the Karl Fischer titration. Of these most common watercontent measurements, however, the Karl Fischer titration has become themethod of choice and is now the approach most widely used in thedetermination of water content. The determination of moisture inmaterials such as liquids and solids by the Karl Fischer reaction iswell known and widely used since it was first described by Karl Fischerin Angewandte Chemie 48, pages 394-396 (1935). Numerous publicationshave also described this technique for water determination, andreference is made to a general text by J. Mitchell, Jr. and D. M. Smith,entitled “Aquametry”, published by John Wiley and Sons, 1980. Referenceis also made to a publication by E. Scholz entitled, “Karl FischerTitration,” published by Springer Verlag in 1984.

[0004] In a Karl Fischer reaction, the water to be determined reactswith iodine on a quantitative basis and consequently, the amount ofreacted iodine is a measure of the amount of water present in thesample. The reaction proceeds according to the following expression:

(1) H₂O+SO₂+I₂ =2H⁺+2I⁻+SO₃

[0005] The titration can be run in either protic or aprotic medium, withthe protic medium seeing wider use due to higher sensitivity of thetiter to sample and solvent composition (M. S. Kamat, R. A. Lodder andP. P. DeLuca, Pharmaceutical Research, 1989 6(11) 961-965.). Thereaction in protic media (i.e., alcohol) involves sulfur dioxidereacting with the alcohol to produce an alkyl sulfite in a bufferedmedium using an appropriate base to maintain the solution at the optimalpH. In a coulometric experiment, the iodine is generated electricallyfrom iodine present in the cell. The electric efficiency of this methodis generally 100%, and the amount of water in the sample is calculatedfrom the number of moles of electrons used in the iodine generation. Thecomponents necessary to carry out this reaction have been formulated andare readily available as Karl Fischer reagents. These reagents aredivided into two groups, single-component and two-component systems. Inthe single-component systems, all ingredients (iodine, buffer, SO₂, andsolvent) are in one solution. In the two-component systems, the “vessel”solution contains the buffer, SO₂, and a solvent, while the “titrant”solution contains iodine in a suitable solvent.

[0006] Thus, Karl Fisher reagents are used in several types of analysis.A volumetric analysis using a volumetric reagent determines moisture bymeasuring the volume of the Karl Fischer reagent consumed during theanalysis. A coulometric analysis using a coulometric reagent generatesiodine by passing a current through the reagent and determines themoisture from the amount of current.

[0007] Analytical instrumentation, semi-automating the Karl Fischerassay, is most commonly used to conduct Karl Fischer titrations. Workingmedium (Methanol) is added to the titration vessel and conditioned toequilibrium (end point with a slight excess of reagent) with titrant.The weighed sample is then delivered into the vessel for titration tothe same end point. The amount of water in the sample under test isdetermined using the reagent strength factor (based on instrumentcalibration with material of known water content) and volume of reagentdispensed to reach equilibrium.

[0008] Examples of instrumentation utilizing the Karl Fisher reactionfor determination of water content comprise: 1) Volumetric MoistureMeter, Model KF-100, Mitsubishi Chemical Corporation; 2) Aquastar®Volumetric Titrator, Models V1B and V-200, EM Science; 3) SchottTitroline KF, Schott; 4) Metrohm® Volumetric Karl Fischer TitrationSystems, Models 701, 784, 758, 756, Brinkmann Instruments, Incorporated;5) Orion® Volumetric Karl Fischer Titrators, Models TURBO2™ and AF8,Thermo Orion, Incorporated; and 6) Mettler-Toledo Titrators, ModelsDL53, DL55, DL58, Mettler-Toledo Corporation.

[0009] Accurate moisture content determination measurements using theKarl Fischer titration are contingent on the proper working order of thetitration instrument and the chemical reactions. Successful moisturecontent determinations require that 1) equipment be in proper workingorder, 2) reagents be stable and not depleted, 3) moisture be excludedfrom the system, 4) the anodic reaction produce 100% current yield, 5)the cathodic reaction does not interfere with the titration, and 6) thereaction not be adversely affected by the sample matrix.

[0010] To assure that these criteria are being met, the quality of theanalysis is checked against calibration standards containing knownmoisture content. The correct moisture content determination for thestandards confirms that the Karl Fischer titration analysis is runningproperly, or indicates that a problem exists. A variety of materialshave been proposed as standards for moisture content determinations. Theprincipal requirements of these materials are 1) that they contain astoichiometric amount of moisture that is stable over a wide range oftemperature and humidity, 2) solubility in the Karl Fischer titrationreagents, 3) ease of handling and storage, 4) availability, and 5)uniformity (M. S. Kamat, R. A. Lodder and P. P. DeLuca, PharmaceuticalResearch, 1989, 6(11), 961-965.).

[0011] Many possible calibration standards for Karl Fisher determinationof water have been described. These include: purified water, certifiedwater standards (known water content determined by assay), Aluminumpotassium sulfate, Ammonium oxalate, Citric acid, Ferric ammoniumsulfate, Ferrous ammonium sulfate, Lactose, Oxalic acid, Potassiumcitrate, Potassium sodium tartrate, Potassium tartrate, sodium acetate,sodium bitartrate, sodium citrate, and sulfosalicylic acid (Neuss, J.D., Obrien, and M. G., Frediani, H. A., Analytical Chemistry, 23, 1332[1951]).

[0012] Much effort has been given to making liquid water standardsolutions less hygroscopic. These efforts have not been completelysuccessful, as the water content of the solutions change after theseptum over the solutions has been pierced several times. Water is avery good calibration reagent, but it is difficult to accuratelydispense liquid water into the Karl Fischer titrator. When delivered byvolume, the inaccuracies of the small amount delivered make it difficultto obtain an accurate value. A more accurate measurement is obtainedwhen the liquid water is delivered by weight, but this again presentsdifficulties in dispensing the water into the titrator. Also,degradation and stability of the standard become relevant due to thespecial material handling characteristics that must be considered forcertified liquid calibration media.

[0013] Use of sodium tartrate dihydrate in powder form as a calibrationstandard for KF reactions is known in the art (E. Scholz, Karl FischerTitration-Determination of Water-Chemical Laboratory Practice,Springer-Verlag, N.Y. 1984, T. H. Beasley, H. W. Siegler, R. L. Charlesand P. King, Anal. Chem., 1972, 44, 1833-1840). However, bulk powdercalibration standards are difficult to manipulate, which can result inincreased assay variability due ingress of ambient moisture and residualstandard unaccounted for during sample addition. Another problem withthe sample transfer process of the prior art is dispensing thecalibration standard material into the Karl Fischer titrator. Whentrying to pour the powder material through a funnel into the titrator,some material is lost into the atmosphere or adheres to the samplingfunnel, and thus is not all dispensed into the titrator. To mitigatethis detriment, weighing paper can be rolled to create a funnel, butthis requires operator dexterity. In either case, during the transfer ofthe powder, the titrator is open to the atmosphere, and length of timethe vessel is open is inversely related to the accuracy of thedetermination. Therefore, the prior art method using powder calibrationstandards requires significant analyst time and creates variability inassay results.

[0014] Thus, in its prior art configuration, Karl Fischer titrationswere affected by: 1) sample transfer time, 2) relative humidity in thelaboratory, and 3) material lost in the material transfer. These factorsmake it desirable to have an improved calibration standard reagent. Suchan improved reagent would result in reduced time to load the reagent,provide for more accurate and quantitative transfer, and have lessfluctuation in water content, as compared to the prior art liquid andpowder calibration standards.

[0015] Accordingly, it is an object of this invention to provide aformed tablet calibration standard-reagent for calibrating Karl Fischerreactions for determining water content in a substance. It is anotherobject of this invention to provide an improved process for thedetermination of water in a sample using the Karl Fischer reaction, inwhich the calibration reagent that is employed is a formed tabletcalibration standard-reagent. Said formed tablet calibrationstandard-reagent containing a first component, namely sodium tartratedihydrate, and a second component, namely magnesium stearate.

[0016] A formed tablet calibration standard-reagent would fundamentallyreduce variability in the Karl Fischer assay. Differences due to analysttechnique would be minimized because standard addition is simplified andmore consistent. Cumbersome use of a syringe and injection into thetitration vessel would be replaced with a single hand transfer of thetablet to the vessel through the sample port. Titration methodologywould remain the same in all other aspects with the exception ofinstrument calibration. A formed tablet calibration standard wouldremove the barriers posed by prior art standards which act to deter theautomation of Karl Fischer determination of water content. An automatedKarl Fischer assay employing a formed tablet calibration standard woulddramatically increase productivity in Karl Fisher water determinations.

[0017] These and other objects, features, and advantages will beapparent from the following more particular description of the preferredembodiments of the invention.

BRIEF SUMMARY OF THE INVENTION

[0018] The invention relates to a formed tablet calibrationstandard-reagent for calibrating Karl Fischer reactions for determiningwater content in a substance, said reagent containing a first component,namely sodium tartrate dihydrate, and a second component, namelymagnesium stearate, where the ratio by percent weight of said firstcomponent to said second component is from 99.7:0.3 to 99:1.

[0019] The invention further relates to a method for determining thewater content of a substance using a Karl Fischer analysis wherein, thereaction is calibrated using a calibration standard, the improvementcomprising using said formed tablet calibration standard-reagent. Theinvention further relates to a formed tablet calibrationstandard-reagent kit, comprising a sealed package containing said formedtablet calibration standard-reagent.

DETAILED DESCRIPTION OF THE INVENTION

[0020] As used herein, “Karl Fischer reaction” refers to the chemicalreaction described by equation (1) supra, and all of the embodiments ofthat reaction herein described including those that employsemi-automated instrumentation (described supra).

[0021] The tablets described herein for use in calibrating Karl Fischerreactions may be formed by the customary procedures in the art of tabletmaking. In preparation of these improved reagents, it is preferable thatthe ratio of said first component to said second component is from99.7:0.3 to 99:1. Preferably, the ratio is 99.6:0.4, 99.5:0.5, and99.4:0.6. Most preferably, the ratio is 99.5:0.5.

[0022] The tablet may have a total weight ranging from about 25milligrams to 500 milligrams. Preferably, the total weight is from about50 milligrams to 500 milligrams. Most preferably the total weight isabout 200 milligrams.

[0023] Of all the possible calibration standard materials describedabove, sodium tartrate dihydrate was selected for preparation of theformed tablet calibration standard-reagent. Bulk sodium tartrate isnearly 100% pure and stable for an extended duration, without specialstorage requirements. In addition, sodium tartrate has a knowntheoretical moisture content. These advantages are incorporated into thetablet because of the formulation process. After size exclusion oflarger crystals, the tartrate is compressed to the desired weight. Theweight of the tablet determines its water content. This target amount isdetermined based on the optimum operating range of the titrator beingused. When the water content of the replicate under test falls withinthis range, the variability in the assay is reduced. Since operatingranges vary by manufacturer, range appropriate sized tablets can bedeveloped for optimal results in a specific instrument type.

[0024] The addition of magnesium stearate improved tablet robustness andeliminated capping. The development of tablet formulation includedexperimentation to assess compression and tablet hardness versus releaseof moisture in the Karl Fischer assay. Tablets were formulated with0.25% magnesium stearate; however, tablet production failed. The lack ofsufficient lubricant caused the press to seize during production. Apreferred embodiment was determined to be tablet production with 0.5%magnesium stearate by weight. This will be explained in more detail byreference to the following examples, which are merely illustrative, andnot limiting of the invention.

EXAMPLE

[0025] Sodium tartrate, dihydrate ACS reagent grade, was used as thestarting material to make formed tablets. Crystals were sieved through a#30 mesh screen (Fisher Standard Testing Sieve, 600 micrometer opening),and placed in a common container. Retained material was discarded. Topromote formation of the tablet, magnesium stearate was added to thefiltered crystals by sizing through the same screen, and adding theexcipient to the mixture. The two components, sodium tartrate andmagnesium stearate, were mixed at a ratio of 95.5:05 percent by weightfor 30 minutes using a “tumble” style mixing apparatus to achievehomogeneity.

[0026] The homogeneous mixture was then compressed into formed tabletsusing a common tablet press. A 7-millimeter round tool and die set wasselected to form the tablets. Tablet production was carried outaccording to customary practices in the art. The tablets produced bythis procedure were found to have an average thickness of 0.155-0.160inches, and an average hardness of 1.3 KP.

[0027] The resulting tablets were surprisingly well formed and durable.In previous attempts to form tablets without magnesium stearate, or with0.25% magnesium stearate, the tablets were not well formed and could notbe handled without degradation of the tablets. The hardness of thetablet is known in the art of tablet making to be important to thestructural and functional characteristics of the tablet. Further, it isknown in the art of tablet making that generally the greater the forcethat is applied to the materials to be formed, the greater the hardnessof the resulting tablet. Surprisingly, it was discovered that formationof the tablets of the present invention did not follow thisrelationship. Unexpectedly it was determined that the combination ofsodium tartrate dehydrate with magnesium stearate in the ratio of95.5:0.5 produced tablets of optimal hardness. Tablets formed of thisratio were subsequently determined in Karl Fisher water determinationanalysis to provide results that were closest to the theoretical watercontent of sodium tartrate dihydrate, and therefore are most preferableas a formed tablet calibration standard reagent.

[0028] Surprisingly, the tablets formed with 0.5% magnesium stearatecould be handled and used in the methods of Karl Fisher waterdetermination described herein without crumbling and without the lossesof material to the environment or upon contact with transferringinstruments such forceps or weighing boats. The compact and discretenature of the tablets minimized the handling requirements by theanalyst, which resulted in less time to execute the analysis. The formedtablet calibration standard reagent was superior to the use of powder orliquid standard reagents in that analyst time was not required toaliquot the standard, and in that the transfer process was discrete andexpedient.

[0029] Several experiments have been conducted to show that the formedtablet calibration standard-reagent of the present invention wouldproduce values in Karl Fischer water determinations consistent with theexpected theoretical value for sodium tartrate dihydrate. Consistencywith the expected value is necessary in order for the tablet to beuseful as a calibration standard reagent for Karl Fisher waterdeterminations.

[0030] The formed tablet calibration standard-reagents prepared with 95%sodium tartrate dihydrate and 5% magnesium stearate were analyzed byKarl Fischer analysis. A commercial titration apparatus was used for theassays (Orion® Volumetric Karl Fischer Titrators, Models TURBO2™). Eachmoisture analysis was conducted according to the customary proceduresfor this instrument. Briefly, the instrument is standardized byaccurately weighing by difference approximately 30 mg of a liquidstandard, namely Hydranal® water standard 10.0. The aliquot wasdelivered into the titration vessel and titrated to the end point withKarl Fischer reagent according to customary procedures. The tablets ofExample 1 were then analyzed and water content was determined bycustomary procedure for this instrument. The empirical values observedwere within 5 percent of the theoretical water content calculated forthe sodium tartrate dihydrate tablets.

[0031] A preferred embodiment of the present invention pertains to theuse of the formed tablet calibration standard-reagent for calibratingKarl Fischer wherein the Karl Fisher reaction employs the two-componentreagent system described supra. Surprisingly, it was determined that thetwo-component system has a greater capacity for repeated analyses ascompared to the one component system when using the formed tabletcalibration standard-reagent.

[0032] The invention further relates to a method for determining thewater content of a substance using a Karl Fischer analysis, wherein thereaction is calibrated using a calibration standard, the improvementcomprising using a formed tablet calibration standard-reagent.

[0033] The formed tablet calibration standard-reagent is used tocalibrate the Karl Fischer reaction for determining water content intest substances. In use, the formed tablet calibration standard-reagentreplaces the prior art calibration standard in the method of calibratingthe Karl Fisher reaction and determining the water content of testsamples. Use of the tablet provides several advantages over the priorart standards, including reducing the time and effort required of theanalyst and reducing the time the reaction vessel is open to theenvironment. Further, a formed tablet calibration standard-reagentremoves the barriers posed by liquid and bulk-powder standards tocomplete automation of the assay. Material handling of samples in tabletform is easily manipulated by robotics. The formed tablet calibrationstandard-reagent simplifies the requirements for assay automation, andis particularly well suited for pharmaceutical applications where thesamples for water content determination are often pills or tablets. Thissimplified automation strategy would replace repetitious, tedious, andvariable manual determinations of moisture content.

[0034] The invention further relates to a formed tablet calibrationstandard-reagent kit, comprising a sealed package containing said formedtablet calibration standard-reagent. These new reagents can be employedin kits that are sold to users for the determination of water content.An example is a sealed package containing these new reagents, where thecalibration tablet can be easily removed from the sealed package andintroduced into the Karl-Fischer reaction vessel.

[0035] While the invention has been described with respect to particularembodiments thereof, it will be apparent to those of skill in the artthat variations can be made therein without departing from the spiritand scope of the invention. The intended scope of the invention is to belimited only by the issued claims thereof.

We claim:
 1. A formed tablet calibration standard-reagent forcalibrating Karl Fischer reactions for determining water content in asubstance, said reagent containing a first component, namely sodiumtartrate dihydrate, and a second component, namely magnesium stearate,where the ratio by percent weight of said first component to said secondcomponent is from 99.7:0.3 to 99:1.
 2. The reagent of claim 1, where theratio of said first component to said second component is 99.6:0.4. 3.The reagent of claim 1, where the ratio of said first component to saidsecond component is 99.5:0.5.
 4. The reagent of claim 1, where the ratioof said first component to said second component is 99.4:0.6.
 5. Thereagent of claim 1, where said tablet has a total weight of betweenabout 25 milligrams and 500 milligrams.
 6. The reagent of claim 1, wheresaid tablet has a total weight of between about 50 milligrams to 500milligrams.
 7. The reagent of claim 1, where said tablet has a totalweight of about 200 milligrams.
 8. The reagent of claim 1, where theratio of said first component to said second component is 99.5:0.5, andwhere said tablet has a weight of about 200 milligrams.
 9. In a methodfor determining the water content of a substance using a Karl Fischeranalysis wherein, the reaction is calibrated using a calibrationstandard, the improvement comprising using the formed tablet calibrationstandard-reagent of claim
 1. 10. In a method for determining the watercontent of a substance using a Karl Fischer analysis wherein, thereaction is calibrated using a calibration standard, the improvementcomprising using the formed tablet calibration standard-reagent of claim3.
 11. In a method for determining the water content of a substanceusing a Karl Fischer analysis wherein, the reaction is calibrated usinga calibration standard, the improvement comprising using the formedtablet calibration standard-reagent of claim
 8. 12. A formed tabletcalibration standard-reagent kit comprising a sealed package containingthe formed tablet calibration standard-reagent of claim
 1. 13. A formedtablet calibration standard-reagent kit comprising a sealed packagecontaining the formed tablet calibration standard-reagent of claim 3.14. A formed tablet calibration standard-reagent kit comprising a sealedpackage containing the formed tablet calibration standard-reagent ofclaim
 8. 15. In a method for determining the water content of asubstance using a Karl Fischer analysis wherein, the reaction iscalibrated using a calibration standard, the improvement comprisingemploying the kit of claim
 12. 16. In a method for determining the watercontent of a substance using a Karl Fischer analysis wherein, thereaction is calibrated using a calibration standard, the improvementcomprising employing the kit of claim
 14. 17. The use of a formed tabletcalibration standard-reagent as claimed in claim 1 in a method fordetermining the water content of a substance using a Karl Fischeranalysis wherein the reaction is calibrated using a calibrationstandard.
 18. The use of a formed tablet calibration standard-reagent asclaimed in claim 3 in a method for determining the water content of asubstance using a Karl Fischer analysis wherein the reaction iscalibrated using a calibration.
 19. The use of a formed tabletcalibration standard-reagent as claimed in claim 8 in a method fordetermining the water content of a substance using a Karl Fischeranalysis wherein the reaction is calibrated using a calibrationstandard.